Searching for gravitational waves via Doppler tracking by future missions to Uranus and Neptune

TL;DR

Future ice giant space missions, with their long cruise times, could detect low-frequency gravitational waves and black hole mergers, significantly enhancing current detection capabilities and localization precision when combined with LISA.

Contribution

This paper evaluates the potential of upcoming ice giant missions to detect gravitational waves and black hole mergers, providing sensitivity estimates and detection rate predictions.

Findings

Estimated detection of SMBH mergers with ~0.5 probability per mission

Detection of EMRIs ranging from 0.1 to 100 events

Ice giant missions combined with LISA improve localization by an order of magnitude

Abstract

The past year has seen numerous publications underlining the importance of a space mission to the ice giants in the upcoming decade. Proposed mission plans involve a $\sim$10 year cruise time to the ice giants. This cruise time can be utilized to search for low-frequency gravitational waves (GWs) by observing the Doppler shift caused by them in the Earth-spacecraft radio link. We calculate the sensitivity of prospective ice giant missions to GWs. Then, adopting a steady-state black hole binary population, we derive a conservative estimate for the detection rate of extreme mass ratio inspirals (EMRIs), supermassive- (SMBH) and stellar mass binary black hole (sBBH) mergers. We link the SMBH population to the fraction of quasars $f_\rm{bin}$ resulting from galaxy mergers that pair SMBHs to a binary. For a total of ten 40-day observations during the cruise of a single spacecraft, $\mathcal{O}(f_\rm{bin})\sim0.5$ detections of SMBH mergers are likely, if Allan deviation of Cassini-era noise is improved by $\sim 10^2$ in the $10^{-5}-10^{-3}$ Hz range. For EMRIs the number of detections lies between $\mathcal{O}(0.1) - \mathcal{O}(100)$. Furthermore, ice giant missions combined with the Laser Interferometer Space Antenna (LISA) would improve the localisation by an order of magnitude compared to LISA by itself.